The measurement of one dimensional planar arrays that are electrically long is currently a difficult task. The accurate measurement of these antenna patterns is limited by the large antenna range requirements for the far field ranges, by the number of near field points that must be measured on a near field range, and by the great size and surface quality requirements of the reflector and feed for compact ranges. This patent application describes the use of applying a quadratic and higher phase correction factors to the phased array and measuring the partially corrected pattern at a distance much shorter than the usual far field distance.
The errors that result from this approach are analyzed as a function of range, the antenna electrical length, the sidelobe level, and the degree of correction which was applied. Both computer simulation and analysis are used to examine the limitations of this approach.
Investigations are currently underway to find methods for mounting large, lightweight, phased arrays on the fuselage of aircraft for airborne surveillance. Such arrays tend to be very long in one dimension, but moderate in size in the other. Because of the physical size of these antennas, the measurement of their basic antenna patterns is very difficult. For example, the far field distance for an antenna with a 10'.times.100' aperture at C-Band would be about 23 miles. By the far field range criteria of R.gtoreq.2D.sup.2 /.lambda. (where R is the distance from the antenna in feet and D is the aperture of the antenna in feet) for a very low sidelobe array, the distance would be much greater if more accuracy in the measurement is desired. The use of compact ranges for an antenna of this size would be extremely impractical. One approach in measuring the far field patterns using a conventional antenna range is based on the fact that in one dimension, the smaller one, the far field range antenna is well within the Fresnel field, but an accurate pattern can only be taken by putting in a phase correction over the aperture. Since the phase correction is only accurate at one angle, repeated corrections are therefore required to obtain a good approximation of the antenna pattern for the far field. In the present invention, the first higher order correction factors are examined, quadratic alone and quadratic and cubic. An analysis of the residual error is given and from that analysis, an estimate of the range of angles over which a particular correction factor gives a small error is found. The range space of angles over which the antenna pattern is measured when R.sub.o is equal to infinity, will determine the required number of resets of the phase shifters to narrow the error factor to an acceptable level.
Thus, it may be seen that at the present time very large phased array antennas cannot be accurately or conveniently measured by present prior art current measurement techniques. The purpose of the present invention is to provide accurate far field antenna pattern measurements using a much smaller range by the application of phase correction to the phased array.